A major health concern for aging adults is their susceptibility to compromised balance and a greater risk of falling. Aging also results in profound structural and functional changes in skeletal muscles. This project will relate the properties of muscles controlling the ankle joint to balance performance in healthy young (21-35 yrs) and older (65-80 yrs) humans. In Specific Aim (SA) 1, ankle joint torque and muscle EMG activity will be measured during isometric and dynamic MVC contractions on a Biodex dynamometer. MRI and ultrasound measurements from the soleus (SO), gastrocnemius (GA) and tibialis anterior (TA) muscles will be combined with the torque data to determine the muscle contractile and elastic characteristics. It is expected that muscles of the older subjects will have slower contractile properties and stiffer elasticity than those of the younger subjects. In Specific Aim 2, the balance characteristics of the same subjects will be measured in quiet stance, static leaning, anterior-posterior (AP) sway, and one arm reaching conditions. A Qualysis motion capture system will measure body kinematics to compute center of mass (COM) motion, an AMTI force plate will measure ground forces and center of pressure (COP) motion, and SO, GA and TA muscle EMG activity will be recorded. COM and COP data will be used to compute motion variability and stability boundaries to assess differences in balance control between the young and old subjects. It is expected that the older subjects will exhibit increased postural variability and reduced temporal margins when driven closer to stability boundaries, and show delayed anticipatory and corrective EMG responses during postural perturbations. Using regression models, the muscular property (SA 1) and balance (SA 2) results will be correlated to assess the impact of age-related changes in muscle properties on balance. In Specific Aim 3, the muscle properties from SA 1 will be used in a musculoskeletal model to simulate the COM motion during the AP sway condition. The model COM motion will be compared to the experimental data from SA 2 to evaluate the accuracy of the musculoskeletal model. The development of this model will permit further investigation of the effect of muscular properties on balance. This project is potentially important to develop successful balance training protocols designed to ameliorate the effects of age-related deficits in balance. Such balance training could reduce the risk of falling and improve the quality of life for elderly adults. [unreadable] [unreadable]